The proposal seeks to understand the mechanisms by which cytotoxic T lymphocytes recognize and kill specific target cells. The work is based upon the availability of large quantities of purified human histocompatibility antigens and upon the development of an in vitro system in which mouse cytotoxic T lymphocyte precursors are induced to differentiate into cells expressing cytotoxicity upon being presented with the purified antigens reconstituted into phospholipid vesicles. Available data indicate that this induction is antigen specific, and that the CTL formed specifically recognize the appropriate antigens on the target cell surface. In order to understand recognition in this system, a binding assay will be developed in which cytotoxic T lymphocytes are allowed to interact with radiolabeled histocompatibility antigens in phospholipid vesicles. This will allow assessment of the physico-chemical characteristics of the binding. Antigen-containing vesicles will also be used in an attempt to inhibit target cell lysis. By comparison with the known binding characteristics of CTL to intact target cells, it will be possible to distinguish the initial binding event from subsequent steps. A second part of the proposal seeks to examine the cytotoxic mechanism by using histocompatibility antigen-containing vesicles or erythrocyte ghosts to which these vesicles have been fused, as model targets. These targets will be used to assess the influence of target cell membrane composition and organization on susceptibility to lysis. In addition, by using internal markers of various sizes, the size of the initial lesion will be determined. The availability of the present system, and the considerable precedent for the method, suggests that immobilized histocompatibility antigens would be excellent immunoadsorbents for T cell antigen-binding molecules (receptors). By using biosynthetically labeled T cells, attempts will be made to purify and characterize molecules which specifically bind to the antigen. Finally, the production of antisera specific for differentiated T cells and the use of cell monolayers as adsorbents are contemplated as a means for the isolation of molecules unique to differentiated lymphocytes.